Automatic knitting machines use banks of large numbers of closely spaced latch needles to interlock threads in a series of connected loops to produce a knitted fabric. The latch needle is a flat needle generally with a long shaft having, at one end, a small hook with a latch, which latch swivels to open and close the hook.
Generally, in a modem knitting machine, many thousands of latch needles are accurately positioned and maintained in a closely packed parallel array. In the process of knitting a fabric, an activation station activates latch needles by moving them forwards and backwards, parallel to their lengths, so that the hook ends of the activated latch needles move towards and away from threads being woven into the fabric. As a latch needle is moved forwards and backwards, its latch swivels back and forth to alternately open and close the latch needle hook so that the latch needle can catch and hold one of the threads being woven into the fabric, pull it to create a loop of fabric, and then release the thread to repeat the cycle.
In rotary knitting machines the needles in an array are held in a cylindrical geometry and rapidly moved, in a rotary motion, into and out of the activation station. Depending upon the fabric being knitted, different ones of the needles moving through the activation station are activated. In linear knitting machines, latch needles are held in parallel slots in large flat needle beds. The activation station is a type of shuttle that moves rapidly back and forth over the needle bed, activating needles appropriate to the weave of the fabric being knitted.
In both rotary and linear knitting machines, a device called a "selector" determines (hereafter referred to as "selects") whether a needle in the activation station of the knitting machine is to be activated or not. To prevent a needle from being activated, the selector presses on a small protuberance (hereafter referred to as an "activation fin" or "fin") on the shaft of the needle. When pressure is applied to the activation fin by the selector, the needle moves away from an activating mechanism of the activator station and is "deactivated". If the selector does not press on the activation fin, the needle is activated.
The selector presses on the fin of a needle, to deactivate the needle, with a "selector foot". The selector foot has two operational selection positions. In a deactivate selection position, the selector foot presses on the fin of the needle thereby preventing the needle from being activated when the needle passes through the activation station. In an activate selection position, the selector foot does not press on the fin of the needle, thereby allowing the needle to be activated when the needle passes through the activation station. The selector foot is generally switched between the selection positions by displacing the selector foot by a small linear translation or by rotating the selector foot through a small angle.
When a knitting machine is operating, the selector of the knitting machine is set to an appropriate selection position for each latch needle that passes through the activation station of the knitting machine. If the selection positions for two needles that pass consecutively through the activation station are not the same the selector has to be switched from one selection position to the other. Prior art selectors generally use solenoids or piezoelectric bimorph actuators to effect the displacements necessary to switch a selector foot between selection positions. However, using these types of actuators, the time it takes to switch a selector foot between selection positions is too long to match the rate at which modern knitting machines move needles through activation stations.
In order to improve the speed with which prior art selectors operate, prior art selectors generally comprise a multiplicity of selector feet which are operated in parallel. In a selector operating with one selector foot, a decision to switch or not switch the selection position of the selector foot, hereafter referred to as "setting" the selector foot, has to be made and executed for every needle that moves through an activation station. In a selector with N activation feet on the other hand, each foot has to be set once for every N needles that move through the activation station. If the switching time needed to switch a selector foot between selection positions is .tau. secs, a selector with one foot can select 1/.tau. needles/sec, or equivalently, operate at a "decision" frequency of 1/.tau. Hz. A selector with N selector feet in parallel on the other hand, can select N/.tau. needles/sec, i.e. operate at a decision frequency of N/.tau. Hz. Switching times for prior art activation feet are on the order of 10 msecs. By operating approximately 10 activation feet in parallel, prior art selectors are able to operate at decision frequencies of up to about 1000 Hz.
The decision frequencies at which prior art selectors operate limit the rate at which needles can be moved through a knitting machine activation station and therefore limit the rate at which fabric can be produced. In order to increase the rate at which knitting machines produce fabric, it is desirable to have selectors that can operate at frequencies higher than 1000 Hz.